Our goal is to understand the neural mechanisms of visual crowding. Crowding is a stimulus interaction phenomenon that results in a blurring degradation of visual perception. Without moving the eyes only a small area of the visual scene gives rise to a clear visual experience. This area is known as the functional visual field. Within it we can select and scrutinize items, recognize and define their features and interrelated parts. Outside of it we have difficultly forming distinct, vivid impressions of the objects surrounding us. Our hypothesis is that the transition across the functional visual field boundary is governed by visual spatial crowding. Crowding plays a major role in limiting the remaining visual function of people with central visual field loss, e.g., macular degeneration, because it dramatically limits the use of the peripheral visual field for daily activities such as reading. Reading is especially slow for people with low vision from central field loss because of the dominance of crowding in the near periphery. A goal of our research is to understand how crowding results from the convergence of information in neuronal receptive fields. We have identified extrastriate cortical area V4 as an area likely to be involved in the interactions of letter-like visual stimuli that lead to crowding phenomena. The proposed research examines five principle questions about the neural mechanisms of crowding. 1) Are the spatial asymmetries of crowding explained by the neural rescaling of spatial information known as the cortical magnification factor? 2) Does organization with respect to the center of the receptive field play a critical role in determining crowding interactions at the neuronal level? 3) Does stimulus size play a major role in crowding within the receptive field or is the primary factor spacing? 4) Does attention alter crowding by changing the basic sensitivity within the crowded area or is it needed to readout information without altering the basic sensitivity? 5) Are the influences of the suppression surround critical in modulating the crowding interactions within the classic receptive field? These issues will be investigated using neurophysiological techniques to study the activity of single cortical neurons while subjects perform visual fixation and visual attentive discrimination tasks. PUBLIC HEALTH RELEVANCE: Millions of people now suffer from low vision disorders that limit central vision;these disorders increase within the aging population. The goal of this research program is to provide an understanding of the neural mechanisms of visual crowding that limit the use of the peripheral vision in normal daily activities such as reading and driving. Understanding of the basic neural mechanism can lead to enhanced clinical approaches, suggest alternate rehabilitative methods, and provide for better, more precise evaluations of visual health.